1. Field of the Invention
The present invention relates to an image processing method and image processing apparatus and, more particularly, to an image processing method and image processing apparatus which generate a conversion characteristic look-up table between input and output devices, that is referred to in a color matching process.
2. Description of the Related Art
FIG. 1 is a conceptual view of general color matching between different devices. Referring to FIG. 1, when RGB or CMYK data as a device dependence image data value is input, the input data is converted into XYZ or L*a*b* data (under the PCS condition to be described later) on a device independence color space by a source profile Src. The input data converted into data on the device independence color space undergoes color space compression so that all colors fall within the color gamut of an output device. This compression is performed because an output device does not express colors outside its color gamut. After color space compression, the input data is converted from the data on the device independence color space into R′B′G′ or C′M′Y′K′ data on a color space dependent on the output device. This color space compression and conversion into the output device values are performed by a destination profile Dst.
In conventional color matching, a reference white point and ambient light are fixed by International Color Consortium (ICC) and the like. For example, in profiles specified by ICC, the Profile Connection Space (PCS) that connects profiles is defined by XYZ values and L*a*b* values of the D50 reference. For this reason, upon viewing an input document or printout under a light source with the D50 characteristics, correct color reproduction is guaranteed. Under light sources with other characteristics, however, correct color reproduction is not guaranteed.
Upon observing an identical output product under different light sources, XYZ values relative to the output product observed under the different light sources look different from each other. Various transform methods are known for predicting XYZ values under different light sources. As such transform methods, (1) ratio conversion, (2) Von Kries transform, and (3) a prediction formula based on a color appearance model will be described below.
(1). Ratio Conversion
The ratio conversion is a method of performing W2/W1 ratio conversion so as to convert XYZ values under reference white point W1 into those under reference white point W2. When this method is applied to an L*a*b* uniform color space, L*a*b* values under reference white point W1 match those under reference white point W2. For example, let (X1,Y1,Z1) be the XYZ values of an output product under reference white point W1(Xw1,Yw1,Zw1), and (X2,Y2,Z2) be the XYZ values of an output product under reference white point W2(Xw2,Yw2,Zw2). Then, according to the ratio conversion, we have:X2=(Xw2/Xw1)·X1Y2=(Yw2/Yw1)·Y1Z2=(Zw2/Zw1)·Z1
(2) Von Kries Transform
The Von Kries transform is a method of performing W2′/W1′ ratio conversion on the human color perception space PQR so as to convert XYZ values under reference white point W1 into those under reference white point W2. When this method is applied to an L*a*b* uniform color space, L*a*b* values under reference white point W1 do not match those under reference white point W2. For example, let (X1,Y1,Z1) be the XYZ values of an output product under reference white point W1(Xw1,Yw1,Zw1), and (X2,Y2,Z2) be the XYZ values of an output product under reference white point W2(Xw2,Yw2,Zw2). Then, according to the Von Kries transform, we have:
      [                                        X            ⁢                                                  ⁢            2                                                            Y            ⁢                                                  ⁢            2                                                            Z            ⁢                                                  ⁢            2                                ]    =                              [                      M                          -              1                                ]                ⁡                  [                                                                                          P                    2                                                        P                    1                                                                              0                                            0                                                                    0                                                                                  Q                    2                                                        Q                    1                                                                              0                                                                    0                                            0                                                                                  R                    2                                                        R                    1                                                                                ]                    ⁡              [        M        ]              ⁡          [                                                  X              ⁢                                                          ⁢              1                                                                          Y              ⁢                                                          ⁢              1                                                                          Z              ⁢                                                          ⁢              1                                          ]      where P1, P2, Q1, Q2, R1, and R2, have a relationship expressed by:
      [                                        P            1                                                            Q            1                                                            R            1                                ]    =                              [          M          ]                ⁡                  [                                                                      Xw                  ⁢                                                                          ⁢                  1                                                                                                      Yw                  ⁢                                                                          ⁢                  1                                                                                                      Z                  ⁢                                                                          ⁢                  w                  ⁢                                                                          ⁢                  1                                                              ]                    ⁢                          [                                                  P              2                                                                          Q              2                                                                          R              2                                          ]        =                  [        M        ]            ⁡              [                                                            Xw                ⁢                                                                  ⁢                2                                                                                        Yw                ⁢                                                                  ⁢                2                                                                                        Z                ⁢                                                                  ⁢                w                ⁢                                                                  ⁢                2                                                    ]            
(3) Prediction Formula Based on a Color Appearance Model
The prediction formula based on a color appearance model is a method of exploiting a human color perception space QMH (or JCH) like CIE CAM97s so as to convert XYZ values under observation condition VC1 (including W1) into those under observation condition VC2 (including W2). Note that Q of QMH represents the brightness; M, the colorfulness; and H, the huequadrature or hueangle. Also, J of JCH represents the lightness; C, the chroma; and H, the huequadrature or hueangle.
For example, let (X1,Y1,Z1) be the XYZ values of a sample under reference white point W1(Xw1,Yw1,Zw1), and (X2,Y2,Z2) be the XYZ values of a sample under reference white point W2(Xw2,Yw2,Zw2). Then, according to the prediction formula based on a color appearance model, conversion is performed such as(X1, Y1,Z1)→[CIE CAM97s forward conversion]→(Q,M,H), or(J,C,H)→[CIE CAM97s inverse conversion]→(X2,Y2,Z2).
The above-described methods are available when the output product is expressed on an ideal medium (a medium on which a white point corresponds to perfect reflection, and a black point corresponds to perfect absorption). In practice, since situations differ depending on media to be used, Japanese Patent Laid-Open No. 2002-094811 discloses a method of matching human color perception between media having different white points or black points. More specifically, when the Von Kries transform is expanded and an effect by a cone response (Pk, Qk, Rk) to a black point under a light source IA and a cone response (Pk′,Qk′,Rk′) to a black point under a light source IB is considered, we have:(P−Pk)/(Pw−Pk)=(P′−P′k)/(P′w−P′k)(Q−Qk)/(Qw−Qk)=(Q′−Q′k)/(Q′w−Q′k)(R−Rk)/(Rw−Rk)=(R′−R′k)/(R′w−R′k)
When this is applied to a method of converting media white and black points to the PCS (D50), a color appearance model which makes human visual perception adaptable to the media white and black points is derived. More specifically, the relationship between an output product (X1,Y1,Z1) on a medium and an output product (X2,Y2,Z2) on the PCS is expressed as follows by using, for example, the Von Kries transform as a chromatic adaptation model. Let MW1(Xmw1,Ymw1,Zmw1) be the medium white point, MK1(Xmk1,Ymk1,Zmk1) be the medium black point, IW2(Xiw2,Yiw2,Ziw2) be the white point D50 on the PCS, and IK2(Xik2,Yik2,Zik2) be the black point on the PCS. Then, we have:
      [                                        X            ⁢                                                  ⁢            2                                                            Y            ⁢                                                  ⁢            2                                                            W            ⁢                                                  ⁢            2                                ]    =            [              M                  -          1                    ]        ⁡          [                                                                                    (                                                            P                                              2                        ⁢                        w                                                              -                                          P                                              2                        ⁢                        k                                                                              )                                ·                                  (                                                            P                      -                                              P                                                  1                          ⁢                          k                                                                                                                                    P                                                  1                          ⁢                          w                                                                    -                                              P                                                  1                          ⁢                          k                                                                                                      )                                            +                              P                                  2                  ⁢                  k                                                                                                                                          (                                                            Q                                              2                        ⁢                        w                                                              -                                          Q                                              2                        ⁢                        k                                                                              )                                ·                                  (                                                            Q                      -                                              Q                                                  1                          ⁢                          k                                                                                                                                    Q                                                  1                          ⁢                          w                                                                    -                                              Q                                                  1                          ⁢                          k                                                                                                      )                                            +                              Q                                  2                  ⁢                  k                                                                                                                                          (                                                            R                                              2                        ⁢                        w                                                              -                                          R                                              2                        ⁢                        k                                                                              )                                ·                                  (                                                            R                      -                                              R                                                  1                          ⁢                          k                                                                                                                                    R                                                  1                          ⁢                          w                                                                    -                                              R                                                  1                          ⁢                          k                                                                                                      )                                            +                              R                                  2                  ⁢                  k                                                                        ]      where P, P1w, P1k, P2w, P2k, Q, Q1w, Q1k, Q2w, Q2k, R, R1w, R1k, R2w, and R2k, have a relationship expressed by:
      [                            P                                      Q                                      R                      ]    =                              [          M          ]                ⁡                  [                                                                      X                  ⁢                                                                          ⁢                  1                                                                                                      Y                  ⁢                                                                          ⁢                  1                                                                                                      Z                  ⁢                                                                          ⁢                  1                                                              ]                    ⁢                          [                                                  P                              1                ⁢                w                                                                                        Q                              1                ⁢                w                                                                                        R                              1                ⁢                w                                                        ]        =                                        [            M            ]                    ⁡                      [                                                                                Xmw                    ⁢                                                                                  ⁢                    1                                                                                                                    Ymw                    ⁢                                                                                  ⁢                    1                                                                                                                    Z                    ⁢                                                                                  ⁢                    mw                    ⁢                                                                                  ⁢                    1                                                                        ]                          ⁢                                  [                                                            P                                  1                  ⁢                  k                                                                                                        Q                                  1                  ⁢                  k                                                                                                        R                                  1                  ⁢                  k                                                                    ]            =                                                  [              M              ]                        ⁡                          [                                                                                          Xmk                      ⁢                                                                                          ⁢                      1                                                                                                                                  Ymk                      ⁢                                                                                          ⁢                      1                                                                                                                                  Z                      ⁢                                                                                          ⁢                      mk                      ⁢                                                                                          ⁢                      1                                                                                  ]                                ⁢                                          [                                                                      P                                      2                    ⁢                    w                                                                                                                        Q                                      2                    ⁢                    w                                                                                                                        R                                      2                    ⁢                    w                                                                                ]                =                                                            [                M                ]                            ⁡                              [                                                                                                    Xiw                        ⁢                                                                                                  ⁢                        2                                                                                                                                                Yiw                        ⁢                                                                                                  ⁢                        2                                                                                                                                                Z                        ⁢                                                                                                  ⁢                        iw                        ⁢                                                                                                  ⁢                        2                                                                                            ]                                      ⁢                                                  [                                                                                P                                          2                      ⁢                      k                                                                                                                                        Q                                          2                      ⁢                      k                                                                                                                                        R                                          2                      ⁢                      k                                                                                            ]                    =                                    [              M              ]                        ⁡                          [                                                                                          Xik                      ⁢                                                                                          ⁢                      2                                                                                                                                  Yik                      ⁢                                                                                          ⁢                      2                                                                                                                                  Z                      ⁢                                                                                          ⁢                      ik                      ⁢                                                                                          ⁢                      2                                                                                  ]                                          
According to the technique described above, a profile can be generated by using white point correction and black point correction corresponding to the medium white point and black point. With this process, color conversion can be performed so as to match the grayscales (color sequences that couple white and black points) on different media on the human color perception space, and the color matching closer to the human color perception can be implemented.
However, the relative color matching using the profile undergone white and black correction of the conventional method has the following problem when the degree of whiteness of the paper white (ground color of the medium) is significantly different between the source side and destination side. Particularly, when the white points of the input and output media are on opposite sides with respect to the L* axis on the L*a*b* color space, the matching precision particularly deteriorates on the gray line.
A case wherein the precision on the gray line deteriorates will be described. For example, assume that a conversion from the input device dependence color space to the device independence color space is performed based on the source profile Src on the input side by the conventional white and black correction method as shown FIG. 2. Assume also that a conversion from the device independence color space to the output device dependence color space is performed based on the destination profile Dst on the output side, as an inverse conversion shown in FIG. 3. The color matching process by the combination of such input and output is performed as shown in FIG. 4. Note that the ordinate represents an L* plane, and the abscissa represents an a*b* plane in FIGS. 2, 3, and 4.
With reference to FIG. 4, reference white point W1(e.g., CMYK=0,0,0,0→L*a*b*=90.1,−0.11,1.69) of the input side is converted into reference white point W(L*a*b*=100,0,0) on the PCS by the source profile Src. W(L*a*b*=100,0,0) is converted into CMYK=0,0,0,0 by the destination profile Dst and output. However, CMYK=0,0,0,0 on the destination becomes, for example, reference white point W2(L*a*b*=95.4,−2.13,−4.50) of the output side before white and black correction, in practice.
As shown in FIG. 4, when the white point is greatly moved by color matching, the gray line (a line which couples the white point and black point) also moves greatly, therefore the matching precision of the image to be output in practice with respect to the input image data deteriorates.